1. Field of the Invention
This invention relates generally to the field of magnetic resonance (MR) imaging (MRI) utilizing NMR phenomena. It is particularly related to enhancement of MR imaging data acquired using hybrid imaging (HI) MR data acquisition sequences which include T2 and additive noise degradation effects.
2. Related Art
Magnetic resonance (MR) imaging techniques are now extensively used for noninvasive investigations of living matter in medicine and biology. Researchers in different MR groups are trying to further improve the performance and the accessibility of MR image machines through lower cost, shorter scan time, higher resolution and contrast, higher signal/noise ratio and fewer artifacts. [L. Kaufman, L. E. Crooks and J. Carlson, "Technology Requirements for Magnetic Resonance Imaging System," in Proceedings of Technology Requirements of Biomedical Imaging, IEEE Computer Society Press, May 1991] Among these, reducing imaging time while maintaining image quality has been a very active topic.
In 1978, Mansfield et. al. demonstrated echo planar imaging (EPI) [P. Mansfield and P. G. Morris, "NMR Imaging in Biomedicine," in Advances in Magnetic Resonance, Edited by J. S. Waugh, Academic Press, New York, 1982]. The basic concept behind EPI is that successive spin echoes can be used to encode position information using just a single shot (i.e., a single NMR RF excitation data acquisition sequence). Because of high requirements on gradient coils and power supplies for achieving rapid echo train generation and some other problems, various hybrid imaging (HI) approaches, incorporating aspects of both conventional two-dimensional (2-D) FT imaging and EPI, have been proposed. [Hennig et al. J. Hennig, A. Nauerth and H. Friedberg, "RARE Imaging: A fast imaging Method for Clinical MR." Magne, Reson, Med., Vol. 3, pp. 823-33, 1986; Van Uijen et al. C. M. J. Van Uijen, J. H. Den Boef and F. J. J. Verschuren; Haacke et al. E. M. Haacke, F. H. Bearden, J. R. Clayton and N. R. Lingar, "Reduction of MR Imaging Time and Hybrid Fast Scan Technique," Radiology, Vol. 158, pp. 521-29, 1986; and others ] These techniques use multiple (M) excitations and after each excitation, multiple (N) echoes are used to encode positional information. HI techniques are far less demanding on hardware and thus can be used to decrease imaging time without the cost and technical constraints of EPI.
Since in EPI and HI, phase encoding measurements acquired at different echo times are used to form an image, there are inherently T2 distortions in the acquired data along the phase encoding direction. Depending upon the phase encoding schemes used and the object under the study, loss of spatial resolution and/or contrast may be introduced. [R. T. Constable and J. C. Gore, "The Loss of Small Objects in Variable TE Imaging: Implications for FSE, RARE, and EPI." Magne, Reson. Med., Vol. 28, pp. 9-24, 1992; D. A. Ortendahl, L. Kaufman and D. M. Kramer, "analysis of Hybird Imaging Techniques," Magne, Reson. Med., Vol. 26, pp. 155-73, 1992] Furthermore, there are discontinuities in the frequency response of the effective T2 distortion filter and these discontinuities generate ringing artifacts in the image. Techniques such as inverse filtering have been tried to reduce those T2 effects, based on some prior knowledge about the T2 values of the objects under study. The success of this approach is often limited by lack of knowledge about the T2 values and the existence of measurement noise. The problem of ringing artifacts caused by local discontinuities in the frequency response function of the T2 filter have not yet been successfully addressed.
It is well known that a Weiner filter performs better for image restoration in a noisy environment. [R. C. Gonzalez and P. Wintz, Digital Image Processing, Addison-Wesly Publishing Company, 1988; A. K. Jain, Fundamentals of Digital Image Processing, Prentice Hall, Englewood Cliffs, N.J. 07632, 1989]
The techniques of linear and nonlinear prediction have also been used to reduce truncation artifacts in MR imaging by several people. [M. R. Smith, S. T. Nichols, R. M. Henkelman and M. L. Wood, "Application of Autoregressive Moving Average Parametric Modeling in Magnetic Resonance Image Reconstruction," IEEE Trans. Med. Image., Vol. 5, pp. 132-39, 1986; J. F. Martin and C. F. Tirendi, "Modified Linear Prediction Modeling in Magnetic Resonance Imaging," J. Magn. Reson., Vol. 82, pp. 392-99 1989; E. M. Haacke, Z. Liang and S. H. Izen, "Super Resolution Reconstruction through Object Modeling and Parameter Estimation," IEEE Trans. ASSP., Vol. 37, pp. 592-95, 1989; H. Yan and J. Mao, "Data Truncation Artifact Reduction in MR Imaging Using A Multilayer Neural Network," IEEE Trans. Med. Imag., Vol. 12, pp. 73-77, 1993]